[FEATURE] Add panel-level annotations to the data model

[nicolastakashi]

What this does

This adds an optional annotations block to PanelSpec so annotations can be set per panel. It follows the two-level annotation design being discussed in perses/perses#3001. The dashboard-level half of that design already shipped, and this PR is the data model for the panel-level half.

type PanelAnnotations struct {
    // Enabled toggles the display of dashboard-level annotations on this panel.
    // nil is treated as enabled (default on).
    Enabled     *bool
    // Definitions holds annotations defined locally on this panel, reusing AnnotationSpec.
    Definitions []AnnotationSpec
}

Enabled is a pointer so an omitted value means "on", which is the default-on behavior from the design. Definitions reuses the existing AnnotationSpec type instead of a new parallel shape.

Scope

This is the data model only. It does not touch backend validation, the frontend consumer hook, the panel editor, or Grafana migration, which I will send as separate follow-ups to keep this one small. I opened it as a draft because the shape is still under discussion on #3001, and I would rather agree on that before building the layers on top of it.

Mirrors

The Go types are the source of truth. I regenerated the CUE mirror with make cue-gen (not hand-edited) and updated the TypeScript (ts/src/dashboard/panel.ts) and Java (.../dashboard/panel/PanelSpec.java plus a new PanelAnnotations.java) mirrors by hand.

Validation

Panel-local definitions get validated without any extra code. AnnotationSpec.UnmarshalJSON already calls validate(), which requires a non-empty name, and that runs when a definition is unmarshaled inside a PanelSpec. So I did not add validation to PanelSpec itself.

Testing

go/dashboard/panel_test.go covers JSON and YAML round-trips, the default-on case when the block is omitted, and rejection of a definition with an empty name.

• make cue-gen cue-eval: clean
• go test ./go/...: all pass
• gofmt -l go/: clean

One side note

Running make cue-gen on macOS hit a portability issue. The target uses GNU-style sed -i, which fails on BSD/macOS sed, so the import-path rewrite step silently does not run. I worked around it locally. I can send a small separate PR to make that sed call portable if that is useful.

Open questions

Same points I raised on #3001, repeated here for context:

1. Is a single per-panel boolean enough for the toggle, or would you want per-annotation opt-out by name?
2. Should panel-local Definitions be in this first iteration, or a follow-up after the toggle?
3. Nested annotations object as done here, or two flat fields on PanelSpec?